Herpes simplex virus (HSV) infections result in recurrent cold sores, corneal lesions, genital ulcers, and in rare cases, encephalitis. Strains of HSV-1 differ in their pathogenesis and show variation in acute infection, latency, and reactivation in animal models. The McKrae isolate is known to be among the most virulent HSV-1 strains, and also reactivates at a higher frequency than other strains. In contrast, strain KOS, which is commonly used to investigate HSV-1 gene function and pathogenesis, is less neuroinvasive and virulent than many other strains of HSV-1 in animal models of infection. Numerous reports indicate that several genes such as ICP0 and ICP34.5 play pivotal roles in HSV neuropathogenesis and virulence. However, the specific viral genes and genetic variants that modulate the pathological phenotypes of McKrae, KOS, and other HSV-1 strains are largely unknown. Understanding the molecular mechanisms that drive the neuropathogenesis of HSV-1 is critical in controlling herpetic infections and the diseases they cause. We have recently catalogued the majority of the allelic differences between KOS and McKrae via genome sequencing, identifying 1,203 single nucleotide polymorphisms (SNPs) and insertion/deletion events distinguishing the strains. Thus, we are in a unique position to resolve the subset of polymorphisms that lead to the differences in pathogenicity between KOS and McKrae. Our work will provide a platform for future functional, mechanistic characterizations of the allelic variants at the causative loci. The objective of this application is to identify loci within the HSV-1 genome that play a role in the heightened neuroinvasiveness and pathogenesis of the HSV-1 strain McKrae over the strain KOS. To most effectively accomplish this objective, the proposal brings together three laboratories with distinct expertise that will be critical for the success of this project. With this background, the question we are asking is: Which regions of the McKrae genome contribute to its neuroinvasiveness? To address this question we have generated genomic data, and have identified specific replication phenotypes in neural tissues that distinguish KOS and McKrae, demonstrating McKrae?s neuropathogenesis in mice. Our rationale for these studies is that a comprehensive understanding of the molecular events in HSV-1 pathogenesis may facilitate the development of new therapeutic interventions to prevent or treat HSV-mediated diseases. The following specific aims are proposed: Specific Aim 1: Identify regions of the HSV-1 genome linked to neuroinvasiveness. Specific Aim 2: Fine map, and functionally characterize loci contributing to HSV-1 neuroinvasiveness. At the completion of this project, we will identify a set of specific McKrae alleles that are strongly associated with its neuroinvasive capacity. Findings from our research are expected to positively impact our understanding of mechanisms used by HSV-1 to enhance its pathogenesis.